The present invention relates to a wavefront sensing device and in particular to a device that senses the deviation in shape of an optical wavefront from its ideal. The wavefront device is particularly, but not exclusively, suitable for use in adaptive optics systems and in confocal microscopes.
An optical system can be considered to be performing at its best when it is free of aberrations. Aberrations in the form of wavefront distortions are particularly problematic and are the cause of degradation in the performance of many telescope and microscope imaging systems. Adaptive optical systems are designed to correct for such aberration and wavefront sensing devices such as the Shack-Hartmann interferometer can be employed to measure the flatness (or deviation from the ideal) of an optical wavefront in the optical system. Once a measure of the flatness of the optical wavefront is known, the optical system can be adapted to correct for the measured aberration.
The Shack-Hartmann wavefront sensor consists of an array of simple lenses and a respective array of quadrant photodetectors. One element of such an array consisting of a lens 1 and a photodetector 2 is shown in FIG. 1. Tilt of the wavefront incident on the lens 1 is translated into a shift in the position of the focussed spot S in the plane of the photodetector 2. The extent of any tilt of a wavefront can therefore be quantified through a suitable combination of signals from the quadrant photodetector 2. With the Shack-Hartmann wavefront sensor the array of such elements is used to measure the local tilt or first differential of the wavefront across a complete aperture and thereby hopefully to reconstruct the complete input wavefront shape.
Wavefront sensing devices are also known for measuring the local defocus or local laplacian of a wavefront. FIG. 2 shows diagramatically one element of a conventional wavefront sensing array that provides a measure of the local defocus of an input wavefront. The element again includes a simple lens 1 but this time two finite sized photodetectors 3, 4 are positioned one in front of the focal plane X of the lens and one behind the focal plane of the lens. This is most conveniently achieved by including a beam splitter 5 with the two detectors 3,4 in orthogonal planes. If the input wavefront is curved, as shown in FIG. 2, then the focal spot will move into focus at one of the detectors and will move out of focus at the other of the detectors. Differential detection gives a signal related to the amount of curvature in the input wavefront.
In a further refinement the two detectors 3,4 can be replaced by a single finite sized detector and the lens 1 replaced by a varifocus lens used to move the focal plane behind and in front of the detector. This refinement of the wavefront sensing element shown in FIG. 2 is the basis of a Roddier curvature sensing device. As in the case of the Shack-Hatmann sensor, in practice an array of such elements would be employed to measure the local wavefront across a complete aperture to thereby reconstruct the complete wavefront shape.
With the present invention a wavefront sensing device is provided comprising a fixed focusing element; at least one detector located at or near the plane of focus; a positive bias wavefront modulating mask and a negative bias wavefront modulating mask each adapted to produce a light point in the plane of focus when illuminated, the difference in intensity between the light points generated by the positive and negative wavefront modulating masks being representative of the deviation of the wavefront shape of the illumination from its ideal.
Ideally, a single wavefront modulating mask is provided that acts as both the positive bias wavefront modulating mask and the negative bias wavefront modulating mask and a pair of spatially separated detectors are provided whereby the single wavefront modulating mask generates a pair of spatially separated light points the difference in intensity between the pair of light points being representative of the deviation of the wavefront shape of the illumination from its ideal.
It should be understood that in the context of this document reference to the detectors being located in or near to the plane of focus is intended as reference to the plane in which an ideal wavefront would come into focus.
Preferably, the wavefront modulating mask is a binary phase mask such as an off axis Fresnel zone plate. Alternatively, the wavefront sensing mask may be a binary polarisation or binary amplitude mask.
In a preferred embodiment, the wavefront modulation mask is patterned with a binarised version of a fraction of the desired wavefront shape to be sensed. The patterning of the binary wavefront modulating mask may be dependent on the desired wavefront shape of the incident illumination defined by reference to one or more Zernike polynomial modes. With this embodiment, ideally the binary wavefront modulating mask is patterned to simultaneously produce a plurality of spatially separated pairs of light points each pair being representative of a different Zernike polynomial mode.
Alternatively, the wavefront modulating mask may be in the form of a spatial light modulator that sequentially forms different biased masks. The different masks may be the positive.and negative bias masks mentioned earlier. In a preferred embodiment the spatial light modulator cycles through a plurality of masks in which each mask may represent a different Zernike polynomial mode (or other system of modes defining wavefront aberration).
The wavefront sensing device may comprise a plurality of wavefront modulating masks, a corresponding array of fixed focusing elements and a corresponding array of pairs of detectors.
The wavefront sensing device may be employed to correct wavefront aberration. Thus, a wavefront correction mask may be provided along with control means for controlling the wavefront correction mask in dependence on the measured aberration measured by a separate wavefront sensing device. With this embodiment the wavefront sensing device may form part of an adaptive optics systems.
In an alternative aspect, the present invention provides an adaptive optics systems comprising one or more fixed focusing elements for imaging a specimen, an adjustable wavefront correction device, a wavefront sensing device as described above and a control device in communication with the wavefront correction device and the wavefront sensing device for adjusting the wavefront correction device in accordance with the wavefront aberration measured by the wavefront sensing device.
In a further aspect the present invention provides a confocal microscope comprising one or more fixed focusing elements for imaging a specimen, a wavefront modulating mask and one or more finite sized detectors for generating a confocal image of the specimen.
The present invention also separately provides a multi-photon confocal microscope comprising one or more fixed focusing elements for imaging a specimen containing one or more fluorescent substances, a wavefront modulating mask and a large area detector whereby a sectioned image of the specimen is generated by virtue of the illuminating light being substantially brighter in the focal region.
With the present invention, a signal is generated when the input wavefront is not perfect with the magnitude of the signal being directly related to the form and magnitude of the input wavefront error. Furthermore, this error signal is relatively large and linear for small input wavefront errors which makes the wavefront sensing device particularly suited to use in adaptive optics systems.